Batch extent

- Adds tests for batch mode
- Tests confirmed that batch mode was generating polylines with wordwide-ish extent (one example: `-122.26387350678164 -38.507738177681574 144.63433781584538 60.13732800531364`)
- Change behavior of batch mode to generate polylines within each city bbox

cc @ianshward @dnomadb @rsudekum